Novel polyamide nanofiltration membrane PEI-(β-CD@g-C3N5)/TMC based on microfiltration substrate for efficient separation of lithium and magnesium

Abstract

With the development of industry, the scarcity of lithium resources is becoming increasingly prominent. Therefore, the development of high-performance nanofiltration (NF) membranes for lithium extraction from salt lakes has become crucial. Herein, β-Cyclodextrin (β-CD) and amino-rich graphitic carbon nitride (g-C₃N₅) sol were uniformly mixed to form β-CD@g-C₃N₅, which was then mixed with polyetherimide (PEI) monomer and subjected to interfacial polymerization (IP) reaction with trimesoyl chloride (TMC) on a 0.22 μm microfiltration (MF) membrane to prepare the PEI-(β-CD@g-C₃N₅)/TMC composite NF membrane. Through thin plate theory and gradient operation pressure filtration tests, it was found that the composite NF membrane prepared on the MF membrane substrate was not only defect-free but also exhibited superior compression resistance. In Li⁺/Mg²⁺ filtration tests, the PEI-(β-CD@g-C₃N₅)/TMC membrane showed superior NF performance, with a Li⁺/Mg²⁺ selectivity coefficient of 38.85 and a permeability of 8.91 L·m⁻²·h⁻¹·bar⁻¹. This was attributed to the effect of β-CD@g-C₃N₅, which could thin the separation layer, slightly enlarge and more uniformly distribute the pores, increase hydrophilicity, and enhance positive charge; meanwhile, the cavity structure provided by β-CD helped to effectively separate Li⁺/Mg²⁺. Furthermore, under conditions of mixed salt solutions with different Mg²⁺/Li⁺ mass ratios and 80 h of continuous filtration, the membrane exhibited excellent stability. This work introduces innovative membrane materials and design concepts to the field of lithium extraction via NF, potentially paving the way for the industrial application of NF technology.